We propose to continue functional genomic studies on the model eukaryote Saccharomyces cerevisiae and will investigate a proposed function for subtelomeric DNA that comprises approximately 7 percent of its genome. Reciprocal recombination (crossover or chiasma formation) between homologs is essential for promoting chromosome segregation at the first meiotic division. However, rare chiasmata near chromosome ends do not promote segregation and could interfere with the development of functional chiasmata elsewhere on the chromosome, thereby preventing proper segregation. Such inappropriate recombination has been suggested as a possible cause of Down and other aneuploid human syndromes. During the past funding period studying functional genomics of S. cerevisiae chromosome I, examination of meiotic reciprocal recombination within both subtelomeric regions revealed low recombination rates and an absence of meiotic recombination-inducing double strand break sites. We investigated the cause of the low rates and found that they were almost entirely dependent on DNA sequence. Based on these studies, we propose that subtelomeric sequences function to insure that meiotic recombination does not occur near chromosome ends. This inhibition insures that crossovers will occur where they promote segregation. To investigate this hypothesis, we propose a genomic approach to determine whether all yeast subtelomeric DNAs show low rates of meiotic reciprocal recombination. The ends and subtelomeres of all chromosomes will be marked and rates of recombination within these regions analyzed and compared to rates on the rest of the chromosome. To confirm there are no telomere position effects (TPE) on meiotic recombination in the terminal 4 kb, the extreme left end of this chromosome I will be analyzed in mutants that relieve TPEs on transcription. Next, to directly demonstrate that subtelomeric regions are required for proper meiotic segregation, segregation fidelity will be examined in homozygous mutants missing all chromosome I subtelomeric DNA. These studies will use conventional assays and new methodology that will be broadly applicable to studying meiotic chromosome transmission fidelity. To investigate the mechanism by which subtelomeric recombination is suppressed, sections of subtelomeric DNA will be tested to determine whether they contain specific sequences that inhibit recombination over distances that extend beyond their length. If such a sequence is found, it will be characterized and the search for genes that encode specific DNA binding proteins initiated. Finally, we will initiate studies to test the proposed model for why subtelomeric recombination does not promote segregation. These studies will lead to a greater understanding of the genomic elements that insure proper chromosome distribution during meiosis and may be important in our search for ways of preventing birth defects due to aneuploid syndromes.